1. Field of the Invention
The present invention relates to an image display device such as a liquid-crystal display (LCD) device and a plasma display device. More particularly, the invention relates to an image display device having a display section including pixels arranged in a matrix array, and a wiring terminal section where a driver IC (Integrated Circuit) or driving semiconductor device is mounted. Mounting electrodes for mounting the driver IC and testing electrodes for testing the display section for defects are formed in the wiring terminal section.
2. Description of the Related Art
In recent years, the LCD device has been extensively used as a high-resolution display or image display device. In general, the LCD device comprises a substrate (which will be referred to as the TFT substrate) on which switching elements such as thin-film transistors (TFTS) are formed, another substrate (which will be referred to as the CF substrate) on which a color filter, a black matrix and so on are formed, and a liquid crystal layer disposed between the TFT substrate and the CF substrate. The alignment of the liquid crystal molecules is changed by the electric field applied across the electrodes formed on the TPT substrate and that formed on the CF substrate, or across the electrodes formed on the TFT, thereby controlling the amount of transmitted light in each pixel to display images.
The section or region of the LCD device used for displaying images, which is termed the “display section”, comprises a plurality of pixels arranged in a matrix array. In the display section, usually, various wiring lines, such as gate electrode lines (or scanning lines), drain electrode lines (or signal lines), common electrode lines, and so on are formed on the TFT substrate.
The LCD device comprises the “wiring terminal section” formed outside the display section, which is a section or region for connecting a driver IC (i.e., a driving semiconductor device) for driving the liquid crystal. In the wiring terminal section, leading lines are formed to electrically connect the driver IC to the wiring lines provided in the display section. The wiring lines formed in the display section will be termed the “internal wiring lines” below. It is typical that the driver IC configured to form a tape carrier package (TCP) is electrically connected to the ends of the leading lines by the tape automated bonding (TAB) method.
By the way, in recent years, the chip on glass (COG) type LCD device, where the driver IC for driving the liquid crystal is directly mounted on the TFT substrate, has been developed and become commercially practical. The COG type LCD device has an advantage that reduction of the thickness and weight can be achieved compared with the conventional LCD device where the driver IC is mounted on the TFT substrate by the TAB method. With the COG type LCD device, a plurality of mounting electrodes or pads, which are used for electrical connection between the leading lines and the driver IC, are formed at midpoints of the respective leading lines. Moreover, a plurality of testing electrodes, which are used for testing the display section for defects, are formed to be adjacent to the corresponding mounting electrodes. The testing electrodes are electrically connected to the corresponding mounting electrodes, in other words, the testing electrodes are formed at one-to-one correspondence to the mounting electrodes.
FIG. 2 is a partial plan view showing the schematic structure of the wiring terminal section of a prior-art COG type LCD device. FIG. 1 is a partial cross-sectional view along the line I-I in FIG. 2.
As shown in FIG. 2, the prior-art COG type LCD device comprises a display section 111 and a wiring terminal section 112. The display section 111 is a section or region for displaying images and includes a plurality of pixels arranged in a matrix array. The wiring terminal section 112, which is provided outside the display section 111, is a section or region on which a driver IC that drives the liquid crystal is mounted and to which a flexible printed circuit (FPC) is connected.
In the wiring terminal section 112, a plurality of leading lines 123 are formed to lead the wiring lines in the display section 111 (i.e., the internal wiring lines) out to the wiring terminal section 112 and to electrically connect the internal wiring lines to the driver IC and the FPC (both not shown). As shown in FIG. 2, these leading lines 123 are extended to the vicinity of the opposite end of the TFT substrate 120 to the display section 111 by way of a mounting region 124 formed in the wiring terminal section 112 on the TFT substrate 120. The mounting region 124 is provided for mounting the driver IC.
In FIG. 2, only 13 of the leading lines 123 are shown. Testing electrodes or pads 126, which are used for testing the display section 111 for defects, are provided at the ends of the respective leading lines 123. Mounting electrodes or pads 125, which are used for mounting the driver IC, are provided on the respective leading lines 123 to be adjacent to the corresponding testing electrodes 126. The mounting electrode 125 and the testing electrode 126 provided on the same leading line 123 are electrically connected to each other by way of the part of the line 123 between these two electrodes 125 and 126.
In FIG. 2, the mounting electrode 125 and the testing electrode 126 provided on the leading line 123 and those provided on its adjacent one are shifted at a predetermined distance along their extension direction (i.e., the vertical direction in FIG. 2), resulting in a zigzag pattern as a whole. The structures of the leading lines 123 and their corresponding mounting and testing electrodes 125 and 126 are the same.
The mounting electrodes 125, all of which are placed in the mounting region 124 for the driver IC, are used to electrically connect the lead lines 123 to the corresponding bumps (not shown) of the driver IC with an asymmetric conductive film (ACF). The testing electrodes 126, all of which are placed outside the mounting region 124, are used to test the display section 111 for defects.
The structure of the leading line 123 is shown in FIG. 1. The line 123 is made of a narrow-belt-shaped conductive film 123a formed on a glass plate 121 of the TFT substrate 120, and a narrow-belt-shaped insulative film 123c formed on the glass plate 121 to cover the conductive film 123a. One end of the conductive film 123a is electrically connected to the corresponding internal wiring line (not shown) in the display section 111 and the other end thereof is extended to the testing electrode 126. Thus, the mounting electrode 125 and the testing electrode 126 are electrically interconnected by way of the conductive film 123a. The conductive film 123a is made of, for example, chromium (Cr) having a low resistivity.
The insulative film 123c, which covers entirely the conductive film 123a, is selectively removed at two positions to form two openings 123cm and 123ci. The mounting electrode 125 is formed to fit into the opening 123cm located on the side of the display section 111. The testing electrode 126 is formed to fit into the opening 123ci located on the opposite side to the display section 111 (i.e., the side of the end of the wiring terminal section 112). Each of the mounting and testing electrodes 125 and 126 is formed to have a plate-like shape whose central position is depressed, where its peripheral part is placed on the insulative film 123c and the remaining part thereof is placed in the opening 123cm or 123ci The electrodes 125 and 126 are in contact with the conductive film 123a by way of the openings 123cm and 123ci, respectively. The electrodes 125 and 126 are made of the same material (e.g., Cr) as the conductive film 123a. 
Cap conductor films 127 and 128 are formed on the mounting and testing electrodes 125 and 126 to cover their almost entire surfaces, respectively. The cap conductor films 127 and 128 are made of, for example, indium tin oxide (ITO) as a transparent conductive material.
As another prior art relating to the present invention, the Japanese Non-Examined Patent Publication No. 2005-121976 discloses an image display device, which comprises the following structure. Specifically, this device comprises internal wiring lines (i.e., wiring lines formed in the display section) and leading lines (i.e., wiring lines formed in the peripheral section), the leading lines being connected in common to one of the internal wiring lines. Some of the leading lines are formed by first conductive lines made of a material (e.g., ITO) having a relatively high corrosion resistance and a relatively high resistivity. The remaining leading lines are formed by second conductive lines made of a material (e.g., a metal) having a relatively low corrosion resistance and a relatively low resistivity.
With this prior-art image display device, when no corrosion occurs in the leading lines, the internal wiring line can be electrically connected to the driver IC (the driving circuit) by way of the leading lines (i.e., the first and second conductive lines) having a relatively low composite resistance. On the other hand, even when corrosion advances in the leading lines, the situation that all the first and second conductive lines (i.e., all the leading lines) are broken down can be prevented. As a result, at least in the initial stage, high display quality is obtainable. Moreover, even after this device is used for a long time, all the leading lines (i.e., the first and second conductive lines) are prevented from being broken down and therefore, the display quality of this device can be kept at a certain level or higher (see FIGS. 5 and 6, paragraphs 0011 to 0012, 0018, and 0026 to 0028).
With the prior-art COG type LCD device shown in FIGS. 1 and 2, the test for detecting a defect in the display section 111 is carried out before mounting the driver IC. During this test, first, the probes of a predetermined testing or inspecting apparatus are contacted with the testing electrodes 126. Thereafter, a predetermined driving voltage is applied to the respective pixels in the display section 111 by way of the testing electrodes 126, thereby testing whether each of the pixels is normally illuminated or not. At this time, due to the low corrosion resistance of Cr, the following problems will occur.
First, because of dirt attached to the testing electrodes 126, wear of the probes of the testing apparatus, attachment of foreign substances to the probes, and so on, minute electric discharge occurs at the contact areas between the testing electrodes 126 and the probes when the driving electrodes is applied. Thus, there is a possibility that change of color of the cap conductor film 128 happens, which leads to the disappearance of the testing electrode 126 made of Cr.
Second, if a cleaning solution is left in the wiring terminal section 112, Cr is turned to an oxide of Cr due to electrochemical reaction to dissolve into the cleaning solution when the driving electrode is applied to the testing electrodes 126 by way of the probes of the testing apparatus. As a result, there is a possibility that the testing electrode 126 made of Cr and the leading line 123 located adjacent thereto disappears.
Third, there is a possibility that the wiring resistance of the testing electrode 126 and the conductive film 123a located adjacent thereto rises due to the corrosion of the Cr films that constitute them.
These phenomena of the corrosion and disappearance of the testing electrode 126 and the Cr films (i.e., the testing electrode 126 and the conductive film 123a) in its vicinity will affect the mounting electrode 125 electrically connected to the testing electrode 126 by way of the conductive film 123a, thereby causing display abnormality.
In addition, with the prior-art image display device disclosed in the Publication No. 2005-121976, although corrosion of the metal wiring lines having a relatively low corrosion resistance is considered, any other thing is not disclosed. Nothing is referred to the testing electrodes 126 of the prior-art COG type LCD device shown in FIGS. 1 and 2. Moreover, of course, no mention is made of the above-described problems occurring in the test of the display section 111 and the countermeasures against these problems.